This invention relates to an anti-lock control device in a 2-channel diagonal brake system.
U.S. Pat. No. 4,744,610 discloses a conventional anti-lock control device of this type.
As shown in FIG. 3, this conventional device comprises a first brake fluid pressure control valve 53a arranged between a first fluid pressure generating chamber 51a of a master cylinder 50 and a front left wheel cylinder 52a, a second fluid pressure control valve 53b arranged between a second fluid pressure generating chamber 51b of the master cylinder 50 and a front right wheel cylinder 52b, a control unit 54 adapted to assess the wheel skid condition and to issue the command for controlling said first and second brake fluid pressure control valves 53a and 53b, and a pressure selecting means 55 adapted to output a pressure corresponding to the lower one of the brake fluid pressures in the left and right front wheels controlled by said first and second brake fluid pressure control valves 53a and 53b.
Said pressure selecting means is provided between the front left wheel FL and the rear right wheel RR, and between the front right wheel FR and the rear left wheel RL, and has a pair of valve elements 56a and 56b for opening and closing the passages between these wheels as well as a pair of coupled pistons 57a and 57b adapted to receive the fluid pressure of the first and second fluid pressure generating chambers 51a and 51b of said master cylinder 50 and that of the wheel cylinders 52a and 52b of said left and right front wheels, respectively, in diagonal directions.
These pistons 57a and 57b are normally in a neutral position, keeping both of said pair of valve elements 56a and 56b in an open state. When one of the brake lines, for example, the line FL-RR, is put in an anti-lock control state and the fluid pressure of the front left wheel cylinder 52a is reduced, the pistons 57a and 57b move according to the pressure reduction, also reducing the fluid pressure of the rear right wheel cylinder 58i a, with the valve element 56a of the one brake line being kept open.
The movement of these pistons 57a and 57b results in the volume of a chamber 59b defined between the valve element 56b and the piston 57b being enlarged, which chamber absorbs the brake fluid pressure of the rear left wheel cylinder 58b in the other brake line (FR-RL).
When one of the brake lines, for example, the line FL-RR fails to operate, the pistons 57a and 57b are either moved toward the position in which the valve element 56b of the other valve line is forced to open, or held in a position in which both of said pair of valve elements 56a and 56b are kept in the open state.
When in such a conventional device, pressure reduction is alternately effected in both brake lines, the vehicle wheels will exhibit the brake fluid pressure characteristics shown in FIG. 4. The fluid pressures of the rear wheels are balanced to the lower one, the fluid pressure of the rear wheel which needs no anti-lock control is also reduced. This results in a reduced rear-wheel braking force, and accordingly a braking distance rather too long.